linux-kernel - Re: uninterruptible CLONE_VFORK (Was: oom: Make coredump interruptible)

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Message-Id: <20100628180416.66F9149A4F@magilla.sf.frob.com>
Date:	Mon, 28 Jun 2010 11:04:16 -0700 (PDT)
From:	Roland McGrath <roland@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: uninterruptible CLONE_VFORK (Was: oom: Make coredump
	interruptible)

> The parent can play with real_blocked or saved_sigmask to block all
> signals except STOP and KILL, use TASK_INTERRUPTIBLE for wait, and
> just return ERESTART each time it gets the signal (it should clear
> child->vfork_done if fatal_signal_pending).

Yes, perhaps.

> We should also check PF_KTHREAD though, there are in kernel users
> of CLONE_VFORK.

There is only __call_usermodehelper, but yes.

> > Bu the way that happens ordinarily is
> > to get all the way back to user mode and reenter with a normal syscall.
> > That doesn't touch the user stack itself, but it sure makes one nervous.
> 
> me too. Especially because I do not really know how !x86 machines
> implement this all.

The only problem I know of off hand is ia64's TIF_RESTORE_RSE (an
arch-specific ptrace detail).  But yes, it would require a careful
reading of all the arch code paths.

> We should also verify that the exiting/stopping parent can never write
> to its ->mm. For example, exit_mm() does put_user(tsk->clear_child_tid).
> Fortunately we can rely on PF_SIGNALED flag in this case.

Right.


Thanks,
Roland
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